The helium balloons probably didn't fall back down to earth as soon as the balloon in the story. But I still think that only 10 km seems pretty remarkable.
Good point, the rate of descent might have made a difference. This sounds like a good experiment. Let's release 10 heavy balloons (just like the ones they used) in pairs of 2 at 5 different locations around the world and track where each of them land. I'm not sure what scientific purpose this would serve but it would at least satisfy my curiosity.